1. Field of the Invention
This invention relates to a lubricating oil composition having improved antiwear performance due to the presence of a metal alkoxyalkylxanthate and a metal thiophosphate.
2. Description of Related Art
Engine lubricating oils require the presence of additives to protect the engine from wear. For almost forty years, the principal antiwear additive for engine lubricating oils has been zinc dialkyldithiophosphate (ZDDP). However, ZDDP must be used in concentrations of 1.4 wt. % or greater to be effective. Since phosphates may result in the deactivation of emission control catalysts used in automotive exhaust systems, a reduction in the amount of phosphorus-containing additives (such as ZDDP) in the oil would be desirable. In addition, ZDDP alone does not provide the enhanced antiwear protection necessary in oils used to lubricate today's small, high performance engines.
The use of metal xanthates in lubricating oil is also known. For example, U.S. Pat. No. 2,335,017 discloses the addition of a metal-containing sulfur compound and a tertiary aliphatic ether or a phenol to a lubricating oil to improve the oil's detergent and anticorrosion properties. Several classes of metallic sulfur compounds are disclosed as being suitable, including metal xanthates of the formula ##STR1## wherein M is a metal and R is an aliphatic or aromatic radical which may contain further substituted atoms or groups such as--O (alkyl). However, there is no mention of a metal thiophosphate being present nor of any improvement in the antiwear performance of the oil.
In addition, certain metal alkoxyalkylxanthates are known. For example, the reaction of nickel methoxyethylxanthate with other compounds has been studied (see Inoro. Chem. Vol. 18, no. 12, pp. 3612-15 (1979)) as has the decomposition of potassium methoxyethylxanthate (see J. Oro. Chem., Vol. 44, no. 10, pp. 1664-9 (1979)). Also, sodium ethoxyethylxanthate and potassium ethoxyethylxanthate are known (see European Patent Application 131,374 and U.S. Pat. No. 3,965,137, respectively). But there is no suggestion of using these compounds in lubricating oils.
However, none of these publications suggest that the antiwear performance of a lubricating oil can be synergistically enhanced when a metal alkoxyalkylxanthate and a metal thiophosphate are present therein.